Ultrasonic pipeline inspection may be performed using a Pipe Inspection Gage (PIG) provided with an array of ultrasound transducers. During inspection the PIG moves through the pipeline and the ultrasound transducers are used to detect ultrasound reflections from the pipeline wall. When the wavelength is sufficiently shorter than the thickness of the pipeline wall, pulse transmission from the PIG results in distinguishable pulse reflections from the inner and outer surface of the pipeline wall. The reflected pulse from the inner surface arrives first, followed by a reflected pulse from the outer surface, which is much smaller, because most of the wave intensity is reflected by the inner surface.
Travel time difference between reflections from the inner and outer surface can be used to determine wall thickness, enabling detection thickness reduction due to wear and corrosion. Furthermore, defects like cracks in the pipe give rise to additional reflections with different travel times, which can be used to detect such defects.
WO 03/083466 describes an apparatus for inspecting the cement casing of a bore hole, comprising a cylindrical array of ultrasound transducers, which makes it possible to produce directed wave fronts. Casing properties may be detected by measuring the decay rate of waves that resonate between the inner and outer surface of a metal pipe in the borehole, in a direction normal to the casing.
WO 03/083466 mentions the problem that conventional inspection techniques assume that the cylindrical array is centered in the pipe, but that the array may in fact be located eccentrically. It is proposed to apply relative delays to the ultrasound transmitted from different transmitters in order to compensate for the eccentricity, thereby effectively centering the generated circular wave front on the pipe axis. The compensation for eccentricity is designed to provide substantially parallel incident wave fronts on the inner surface of the wall, with minimized phase variation of the incident wave front as a function of circumferential position along the wall in a plane of the array. As a result, the adjusted relative time shift of each transmitter substantially compensates for the deviation between the average travel time from all transmitters and the travel time from that transmitter to a corresponding nearest point on the pipe wall.
During use of a pipeline, deposits may accumulate on the inner surface of the pipeline wall. This is called scaling. In oil pipelines for example, a wax-like deposit is formed, often contaminated with particles like sand and corroded metal scraps. With time, such a deposit becomes effectively impenetrable for ultrasound at the frequencies needed to resolve reflected pulses from the inner and outer surfaces of the pipeline wall. Also bends and other deformations can obstruct inspection.